1. Field of the Invention
The present invention relates to a weldment of different materials comprising an aluminum or aluminum alloy (hereinafter both of which are referred to as “aluminum”) sheet and an aluminum-coated steel sheet, and a method of resistance spot welding the weldment.
2. Description of the Related Art
Aluminum is widely used for vehicles and automobiles as light-weight structural materials. Due to various factors including costs, strength and stiffness, aluminum is often used in combination with a steel sheet. The steel sheet is typically joined to each other using resistance spot welding in a simply way. Also, it is required to resistance spot weld aluminum with a steel sheet to provide a joined material.
It is conventionally known that when an aluminum sheet and a steel sheet are directly resistance spot welded, a hard and brittle intermetallic compound is produced at an interface between the aluminum sheet and the steel sheet, resulting in a joined material having a significantly decreased strength.
In order to provide sufficient strength, a coating layer is formed on a steel sheet as disclosed, for example, in Japanese Unexamined Patent Application Publication No. 4-251676.
Also, an insert material is interposed between an aluminum sheet and a steel sheet, and they are resistance spot welded as disclosed in, for example, Japanese Unexamined Patent Application Publication No. 4-253578.
However, the strength is not sufficient after joining, although the coating layer is formed on the steel sheet. In addition, even if the strength is sufficient, the fracture energy is low, whereby no joined material suitable for use in structures such as vehicles and automobiles can be provided.
The resistance spot welding using the insert material is insufficient and not suitable for a large number of joinings.